Especially in retrofit-type of applications, i.e. new products that can directly replace old incandescent lamps, halogen lamps or others, having the same geometric dimensions and fittings (e.g. E14, E27) and using the already existing infrastructure in homes and buildings, driver efficiency and cost are crucial for making LED (Light Emitting Diode)-based light sources a success. For this reason, the idea of connecting strings of LEDs directly to the AC mains voltage, only ballasted by a series resistor, has come up. The efficiency of such a driver is about 80%, which is not very high, but still gives an unmatched form factor and performance to price ratio.
A major drawback of this approach is that the current that is drawn from the AC mains voltage suffers from a rather large third harmonic component. At present this is not an issue because the standard that limits this current does not apply to light sources with nominal input power levels of less than 25 W. In the near future (i.e. in the coming 1-3 years), however, this boundary will be lowered to 5 W and extended to cover also solid state light sources, thereby effectively limiting the overall light output of an ACLED solution.